Two dependencies are complaining. I am using JDK6. A quick peek at the manifests in the dependencies show they require JDK4 as a minimum.

If JDK4 is the minimum version required why the error over JDK6?

I pulled these two bundles from eclipse/plugins.

A bundle could not be resolved because the required execution enviroment did not match the runtime environment. In bundle <o
rg.apache.xerces_2.9.0.v201101211617>
A bundle could not be resolved because the required execution enviroment did not match the runtime environment. In bundle <o
rg.apache.xml.serializer_2.7.1.v201005080400>

Uses constraints are described in the FAQ. The synthetic context bundle is described in the Programmer Guide - search for "synthetic".

Uses constraint failures in resolving the synthetic context bundle are pretty rare - I don't remember seeing one before. So presumably you have transitive dependencies on multiple versions of the same package. The FAQ will give you some ideas for how to diagnose this.